1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine.
2. Description of the Related Art
Exhaust gas purification devices utilizing NO.sub.X occluding and reducing catalysts are known in the art. A NO.sub.X occluding and reducing catalyst absorbs NO.sub.X in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst is lean, and releases the absorbed NO.sub.X and reduces the released NO.sub.X and NO.sub.X in the exhaust gas flowing into the catalyst when the air-fuel ratio of the exhaust gas flowing into the catalyst is a stoichiometric or a rich air-fuel ratio (In this specification, the term "air-fuel ratio of the exhaust gas" means the ratio of the amounts of air and fuel supplied to the engine and exhaust gas passage upstream of a considered point). In this type of the exhaust gas purification device, a NO.sub.X occluding and reducing catalyst is disposed in the exhaust gas passage of an internal combustion engine and absorbs NO.sub.X in the exhaust gas when the engine is operated at a lean air-fuel ratio, thus, NO.sub.X in the exhaust gas is removed. Further, in order to prevent the NO.sub.X occluding and reducing catalyst from being saturated with the absorbed NO.sub.X, the engine is operated at a rich air-fuel ratio for a short period after the operation at a lean air-fuel ratio has continued for a predetermined period. When the engine is operated at a rich air-fuel ratio, exhaust gas of a rich air-fuel ratio flows into the NO.sub.X occluding and reducing catalyst and the NO.sub.X absorbed in the catalyst is released and reduced. Thus, NO.sub.X is not diffused into the atmosphere.
However, the capacity of a NO.sub.X occluding and reducing catalyst for absorbing NO.sub.X becomes lower as the catalyst deteriorates. Therefore, when the NO.sub.X occluding and reducing catalyst has deteriorated to some extent, the catalyst absorbs NO.sub.X in the exhaust gas to its maximum capacity (i.e., the catalyst is saturated by the absorbed NO.sub.X) before the rich air-fuel ratio operation of the engine is carried out. In this case, NO.sub.X in the exhaust gas passes through the NO.sub.X occluding and reducing catalyst without being absorbed therein and diffuses into the atmosphere. Therefore, it is important to determine whether the NO.sub.X occluding and reducing catalyst has deteriorated.
Heretofore, various exhaust gas purification devices having means for determining the deterioration of the NO.sub.X occluding and reducing catalyst have been proposed. One example of this type of exhaust gas purification device is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-232644. The device in the '644 publication is provided with an air-fuel ratio sensor, disposed in the exhaust gas passage downstream of the NO.sub.X occluding and reducing catalyst, which detects the air-fuel ratio of the exhaust gas flowing out from the catalyst. The device determines the deterioration of the NO.sub.X occluding and reducing catalyst based on the change in the air-fuel ratio detected by the air-fuel ratio sensor. More specifically, the device in the '644 publication supplies exhaust gas of a lean air-fuel ratio to the NO.sub.X occluding and reducing catalyst for a period sufficient for the NO.sub.X occluding and reducing catalyst to absorb NO.sub.X in the exhaust gas to its maximum capacity. After the NO.sub.X occluding and reducing catalyst is saturated with the absorbed NO.sub.X, the device in the '644 publication switches the air-fuel ratio of the exhaust gas flowing into the NO.sub.X occluding and reducing catalyst from a lean air-fuel ratio to a rich air-fuel ratio, and determines the deterioration of the catalyst based on the time lapsed, when the air-fuel ratio of the exhaust gas flowing into the catalyst changes to a rich air-fuel ratio, since the air-fuel ratio of the exhaust gas flowing into the catalyst was switched to a rich air-fuel ratio.
As explained before, NO.sub.X absorbed in the NO.sub.X occluding and reducing catalyst is released when a rich air-fuel ratio exhaust gas is supplied to the catalyst. The released NO.sub.X is reduced by reacting with HC and CO in the rich air-fuel ratio exhaust gas. In other words, HC and CO in the exhaust gas are oxidized by the NO.sub.X released from the catalyst. Therefore, the air-fuel ratio of the exhaust gas passing through the catalyst shifts to a lean air-fuel ratio side by this oxidation and the air-fuel ratio of the exhaust gas flowing out from the catalyst becomes leaner than the air-fuel ratio of the exhaust gas flowing into the catalyst. In this case, since the release of NO.sub.X stops when the atmosphere of the catalyst becomes leaner than the stoichiometric air-fuel ratio, the atmosphere of the catalyst becomes near the stoichiometric air-fuel ratio (in the actual operation, an air-fuel ratio slightly higher (leaner) than the stoichiometric air-fuel ratio) when the NO.sub.X occluding and reducing catalyst is releasing the absorbed NO.sub.X. Therefore, the air-fuel ratio of the exhaust gas flowing out from the NO.sub.X occluding and reducing catalyst is maintained near the stoichiometric air-fuel ratio when the absorbed NO.sub.X is released from the catalyst even though the air-fuel ratio of the exhaust gas flowing into the catalyst is a rich air-fuel ratio. When the NO.sub.X occluding and reducing catalyst releases all the absorbed NO.sub.X, since the oxidation of HC and CO in the exhaust gas by the released NO.sub.X stops, the air-fuel ratio of the exhaust gas flowing out from the catalyst becomes an air-fuel ratio same as that of the exhaust gas flowing into the catalyst, i.e., a rich air-fuel ratio.
This means that the time lapsed when the air-fuel ratio of the downstream exhaust gas (i.e., the air-fuel ratio of the exhaust gas flowing out from the catalyst) becomes a rich air-fuel ratio since the air-fuel ratio of the upstream exhaust gas (i.e., the air-fuel ratio of the exhaust gas flowing into the catalyst) has changed from a lean air-fuel ratio to a rich air-fuel ratio is proportional to the amount of the NO.sub.X held in the NO.sub.X occluding and reducing catalyst.
The device in the '644 publication switches the air-fuel ratio of the upstream exhaust gas from a lean air-fuel ratio to a rich air-fuel ratio after the catalyst absorbs NO.sub.X to its maximum capacity and measures the time required for the downstream air-fuel ratio to change to a rich air-fuel ratio. Therefore, this measured time corresponds to a current maximum NO.sub.X absorbing capacity of the NO.sub.X occluding and reducing catalyst and can be used for determining the degree of deterioration of the NO.sub.X absorbing capacity of the catalyst.
International Patent Publication No. WO 94-17291 discloses an exhaust gas purification device similar to that of the '644 publication. The device in the '291 publication is also provided with a NO.sub.X occluding and reducing catalyst in the exhaust gas passage of an internal combustion engine and an air-fuel ratio sensor disposed in the exhaust gas passage downstream of the catalyst. When the engine is operated at a lean air-fuel ratio for a predetermined time, the device in the '291 publication switches the operating air-fuel ratio of the engine to a rich air-fuel ratio in order to release the absorbed NO.sub.X from the catalyst. The rich air-fuel ratio operation of the engine is continued until the air-fuel ratio of the exhaust gas detected by the air-fuel ratio sensor becomes a rich air-fuel ratio. When the detected air-fuel ratio changes a rich air-fuel ratio, the device stops the rich air-fuel ratio operation of the engine and returns to the lean air-fuel ratio operation of the engine. Namely, the device in the '291 publication determines that all the absorbed NO.sub.X has been released when the air-fuel ratio of the exhaust gas flowing out from the NO.sub.X occluding and reducing catalyst becomes a rich air-fuel ratio. Further, the device in the '291 publication also evaluates the deterioration of the NO.sub.X absorbing capacity of the NO.sub.X occluding and reducing catalyst based on the time required for the air-fuel ratio of the exhaust gas flowing out from the catalyst to change to a rich air-fuel ratio.
Though the device in the '291 publication stops the rich air-fuel ratio operation of the engine when a rich air-fuel ratio is detected by the air-fuel ratio sensor, this may cause worsening of the exhaust emission. Since the exhaust gas from the engine reaches the NO.sub.X occluding and reducing catalyst after a certain time required for travelling the exhaust gas passage, a lean air-fuel ratio exhaust gas does not reach the catalyst immediately after the engine operating air-fuel ratio is switched to a lean air-fuel ratio. Therefore, in the device in the '291 publication, a rich air-fuel ratio exhaust gas is supplied to the catalyst even after the catalyst has released all the absorbed NO.sub.X. In this case, since NO.sub.X is not released from the catalyst, the rich air-fuel ratio exhaust gas containing HC and CO passes through the catalyst and HC and CO in the exhaust gas diffuses into the atmosphere. Further, the device in the '291 publication evaluates the NO.sub.X absorbing capacity of the NO.sub.X occluding and reducing catalyst based on the time required for the air-fuel ratio of the downstream exhaust gas to change to a rich air-fuel ratio after the engine operating air-fuel ratio is switched to a rich air-fuel ratio. This length of the time includes the time required for the exhaust gas to travel from the engine to the catalyst. The time required for the exhaust gas to travel from the engine to the catalyst changes in accordance with the engine operating conditions (for example, the velocity of the exhaust gas in the exhaust gas passage). Therefore, if the deterioration of the NO.sub.X absorbing capacity is determined based on the time required for the air-fuel ratio of the downstream exhaust gas to change to a rich air-fuel ratio after the engine operating air-fuel ratio is switched to a rich air-fuel ratio, the deterioration of the NO.sub.X absorbing capacity cannot be evaluated correctly.
A similar problem occurs in the device in the '644 publication since the device also evaluates the deterioration of the NO.sub.X absorbing capacity of the NO.sub.X occluding and reducing catalyst based on the time required for the air-fuel ratio of the downstream exhaust gas to change to a rich air-fuel ratio after the engine operating air-fuel ratio is switched to a rich air-fuel ratio.
Further, even if the NO.sub.X absorbing capacity is correctly evaluated, an ability of the NO.sub.X occluding and reducing catalyst including the ability as an oxidizing and reducing catalyst as well as the NO.sub.X absorbing capacity cannot be evaluated by the devices in the related arts. As explained later, the NO.sub.X occluding and reducing catalyst also acts as an oxidizing and reducing catalyst and this ability largely affects the reduction of NO.sub.X and oxidation of HC and CO. If the ability as the oxidizing and reducing catalyst of the NO.sub.X occluding and reducing catalyst deteriorates, NO.sub.X in the exhaust gas passes through the NO.sub.X occluding and reducing catalyst without reduced even though the NO.sub.X absorbing capacity thereof has not deteriorated. Therefore, it is necessary to evaluate the ability of the NO.sub.X occluding and reducing catalyst including both the ability as an oxidizing and reducing catalyst and the NO.sub.X absorbing capacity.